Microbalances are highly valued for use in scientific and medical laboratories. They are especially useful in measuring weight and weight changes of extremely small samples of material with very high accuracy. For instance, The Perkin-Elmer Corporation, Analytical Instruments Department, of Main Avenue, Norwalk, Conn. 06856, U.S.A., offers microbalance systems products called the "AD-4 Autobalance" and the "AD-6 Autobalance" which are very accurate, and which are capable of weighing small samples of two milligrams and less to a precision of 0.2 micrograms. Also, microbalance devices are especially useful in thermal analysis instrumentation where a minute sample is typically heated, and the dissipation of the sample through evaporation or oxidation is continuously monitored and recorded in terms of the changing weight of the sample. Systems incorporating microbalances which accomplish this thermal analysis objective are referred to as thermogravimetric systems. For instance, The Perkin-Elmer Corporation, the above-mentioned vendor, offers a model TGS-1 thermogravimetric system of this kind.
In such apparatus, such as an autobalance, or a thermogravimetric system, it is common to provide for an automatic electronic counterweight equivalent by introducing an electromagnetic restoring force to the microbalance to bring the microbalance back to a null position while a weighing operation is undertaken, and measuring the signals applied to provide the electromagnetic force as an electrical measurement of the weight. A mechanical tare or counterweight also may be added to the system, if desired. This can improve the accuracy since the electronic counterweight then need not provide such a large electrical signal to restore the microbalance to the null position.
While extremely small samples can be handled with ease in present microbalance instruments, it is desirable, especially in the thermogravimetric systems, to use an initial sample which has an appreciable weight so as to increase the absolute value of the total weight change as heat is applied to the sample, to thereby increase the accuracy with which the results are measured.
Any friction in the bearings of the microbalance is obviously extremely undesirable. For this reason, it has been common to employ a frictionless bearing system for microbalances which incorporates a tensioned torsion wire at the rotational axis of the microbalance, the ends of the wire being connected to a fixed support, and the microbalance being attached at a point intermediate to the ends. As the microbalance rotates about this torsion wire, the wire is twisted, but no real friction is encountered. This ingenious method of suspension resembles that originally used in the d'Arsonval galvanometer.
While the torsion wire rotation axis support of a d'Arsonval galvanometer is often arranged vertically, when the torsion wire is used for the microbalance bearing, it obviously must be arranged horizontally.
Unfortunately, with the horizontal torsion wire microbalance bearing, when a substantial weight is to be measured, (within the microbalance weighing range) the torsion wire tends to sag, and the entire microbalance pivot therefore tends to sag. This is especially true when a physical counterweight is used on the microbalance, but it is also true when employing an electronic automatic counterbalance. Such displacement substantially reduces the accuracy of the instrument. This is particularly true in structures wherein the automatic electronic counterbalance operates by an electromagnetic force applied to the microbalance in the vicinity of the torsion wire bearing, for the sagging of the microbalance bearing upsets the alignment of the rotor and stator of the electromagnetic restoring force apparatus.
Accordingly, it is an important object of the present invention to provide a vertically stable friction-free microbalance structure which avoids the disadvantages of the sagging torsion wire bearing.
Another object of the invention is to provide a vertically stable friction-free microbalance structure in which the accuracy of the microbalance is not adversely affected by the weight of material being weighed, or by the combination of that weight and a physical counterbalance weight.
It is another object of the invention to provide a vertically stable friction-free microbalance structure which provides for greatly improved accuracy and reproducibility of results.
Another object of the invention is to provide a vertically stable friction-free microbalance structure which is singularly free of service and maintenance problems and difficulties.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.